The present invention relates generally to the field of imaging and, more particularly, to the utilization of magnetic resonance imaging to image joint spaces.
Previous techniques utilized to image internal structures of interest in subjects are well known in the art. Generally, prior art techniques utilize computed axial tomographic X-rays, also known as CT scanning, or magnetic resonance imaging (MRI) to afford an internal view of particular regions in subjects undergoing imaging procedures. These prior art techniques may be adequate for image construction of tissue matter such as bone, muscle, brain, spinal cord, veins, arteries and nerves, as well as other tissues. However, particular regions of interest, namely the joint regions and spaces, are not as amenable to prior art techniques of imaging, particularly tissue selective imaging.
Joint spaces are found in such medically important sites as the knee joint, intervertebral discs, the shoulder joint and the hip joint, as well as other joints. These four listed joints, taken together, are the subject of various pathologies that affect millions of patients and require over one million surgical repair procedures each year in the United States.
Despite the enormous medical and economic impact of various joint diseases, there has not been any prior art methods that are amenable to generalized automated use for the imaging of joints that result in unobstructed images or three-dimensional views. Typically, the prior art techniques provide joint images wherein particular angles of viewing the joint are impeded or obstructed by bone, marrow, fat and blood vessels, for example. Prior art methods are typically comprised of operator driven techniques wherein the general image of the joint and surrounding tissue is observed and imaged. The operator then uses a computer-input device to outline and identify components of interest in the joint region in each of a series of successive image slices taken of the joint region. Surrounding tissues that are components of the joint region that are not necessarily of interest are then deleted and the remaining elements of the joint region, in the series of image slices, are computationally stacked and projected. As those skilled in the art can appreciate, this method is a painstakingly slow and subjective process.
The use of X-rays to evaluate joint regions is well known in the art. This typically entails the direct, invasive injection of joints of interests with various X-ray contrast materials. However, dense bone and the injected contrast material often display similar effects on the resultant X-ray image, and as a result, this technique is only of limited use. Further, the cartilage components of the joint may be difficult to view. Therefore, the need for novel and improved techniques and methods of joint imaging will be well appreciated by those skilled in the art of medical imaging.
One approach of particular interest that has been used to image physiological structures is magnetic resonance imaging (MRI). By way of introduction, MRI involves the exposure of tissue to a variety of different magnetic and radio-frequency (rf) electromagnetic fields. The response of the tissue""s atomic nuclei to the fields is then processed to produce an image of the tissue.
More particularly, the tissue is initially exposed to a polarizing magnetic field. In the presence of this field, nuclei exhibiting magnetic moments (hereinafter referred to as spins) will seek to align themselves with the field. The nuclei precess about the polarizing field at an angular frequency (hereinafter referred to as the Larmor frequency) whose magnitude depends upon both the field""s strength and the magnetogyric constant of the specific nuclear species involved.
Although the magnetic components of the spins cancel each other in a plane perpendicular to the polarizing field, the spins exhibit a net magnetic moment in the direction of the polarizing field. By applying an excitation field perpendicular to the polarizing field and at a frequency near the Larmor frequency, the net magnetic moment can be tilted. The tilted magnetic moment includes a transverse component, in the plane perpendicular to the polarizing field, rotating at the Larmor frequency. The extent to which the magnetic moment is tilted and, hence, the magnitude of the net transverse magnetic moment, depends upon the magnitude and duration of the excitation field.
An external return coil is used to sense the field associated with the transverse magnetic moment, once the excitation field is removed. The return coil, thus, produces a sinusoidal output, whose frequency is the Larmor frequency and whose amplitude is proportional to that of the transverse magnetic moment. With the excitation field removed, the net magnetic moment gradually reorients itself with the polarizing field. As a result, the amplitude of the return coil output decays exponentially with time.
Two factors influencing the rate of decay are known as the spin-lattice relaxation coefficient T1 and the spin-spin relaxation coefficient T2. The spin-spin relaxation coefficient T2 represents the influence that interactions between spins have on decay, while the spin-lattice relaxation coefficient T1 represents the influence that interactions between spins and fixed components have on decay. Thus, the rate at which the return coil output decays is dependent upon, and indicative of, the composition of the tissue.
By employing an excitation field that has a narrow frequency band, only a relatively narrow band within a nuclear species will be excited. As a result, the transverse magnetic component and, hence, return coil output, will exhibit a relatively narrow frequency band indicative of that band of the nuclear species. On the other hand, if the excitation field has a broad frequency band, the return coil output may include components associated with the transverse magnetic components of a greater variety of frequencies. A Fourier analysis of the output allows the different frequencies, which can be indicative of different chemical or biological environments, to be distinguished.
In the arrangement described above, the contribution of particular spins to the return coil output is not dependent upon their location within the tissue. As a result, while the frequency and decay of the output can be used to identify components of the tissue, the output does not indicate the location of components in the tissue.
To produce such a spatial image of the region of tissue, gradients are established in the polarizing field. The direction of the polarizing field remains the same, but its strength varies along the x, y, and z axes oriented with respect to the tissue. By varying the strength of the polarizing field linearly along the x-axis, the Larmor frequency of a particular nuclear species will also vary linearly as a function of its position along the x-axis. Similarly, with magnetic field gradients established along the y-axis and z-axis, the Larmor frequency of a particular species will vary linearly as a function of its position along these axes.
As noted above, by performing a Fourier analysis of the return coil""s output, the frequency components of the output can be separated. With a narrow band excitation field applied to excite a select nuclear species, the position of a spin relative to the xyz coordinate system can then be determined by assessing the difference between the coil output frequency and the Larmor frequency for that species. Thus, the MRI system can be constructed to analyze frequency at a given point in time to determine the location of spins relative to the magnetic field gradients and to analyze the decay in frequency to determine the composition of the tissue region at a particular point.
The generation and sensing of the fields required for proper operation of an MRI system is achieved in response to the sequential operation of, for example, one or more main polarizing field coils, polarizing gradient field coils, rf excitation field coils, and return field coils. Commonly, the same coil arrangement is used to generate the excitation field and sense the return field. These rf coils, or antennas, can be of various prior art designs including, surface coils, solenoid coils or multi-coil arrays, such as phased arrays. A variety of different sequences have been developed to tailor specific aspects of MRI system operation, as described, for example, in U.S. Pat. No. 4,843,322 (Glover); U.S. Pat. No. 4,868,501 (Conolly); and U.S. Pat. No. 4,901,020 (Ladebeck et al.).
In order to enhance the utility of MRI systems in imaging various tissues, the administration of pharmaceutical agents is often used to enhance the contrast of particular tissues relative to the surrounding tissues in the images produced. These agents are referred to as contrast agents. Contrast agents act to change the relaxation or return output times of tissues in which they are localized.
A wide variety of contrast agents are known in the art. These are utilized according to the type of imaging to be performed as well as the particular tissue type undergoing the imaging procedures. For example, contrast agent types currently utilized include gastrointestinal, tumor specific as well as intravascular or blood pool agents.
As mentioned previously, although the use of MRI and its various permutations have been successfully utilized to construct images of many tissue and organ types, the production of useful tissue selective images of joint spaces has presented some difficulty. This difficulty in imaging is due to the fact that joint regions are comprised of multiple components including, for example, blood, joint fluids, muscles, tendons, cartilage, fat, bone and the marrow incorporated within the bone. As a result, for effectively every type of magnetic resonance pulse imaging sequence, various component tissues, singly or in combination, result in bright signals in the resultant image. Consequently, prior art methodologies of imaging, in two and three-dimensions, of joint spaces utilizing MRI can be improved upon, wherein particular protocols, herein disclosed, are employed comprising particular pulse sequences in combination with contrast agents resulting in informative images of joint regions and spaces which are unobstructed by various other joint components, such as blood vessels, bone, and marrow for example.
Accordingly, the objective of the present invention is to provide a method that overcomes the previously mentioned obstacles to imaging joints utilizing MRI. More particularly, a method is disclosed whereby components of joint regions, particularly bone and the marrow components incorporated within, are selectively removed singly or in any combination with other components comprising the joint region. This effect of removing bone and marrow contributions to joint images utilizing MRI will be referred to as the xe2x80x9cblack bonexe2x80x9d effect. When put into practice, the method disclosed herein will result in the production of previously unavailable, medically useful images of joint spaces.
It is an additional object of the present invention to provide generalized, automated methods of joint space imaging construction that can performed expeditiously and utilized in conjunction with other various MRI imaging techniques that result in the useful imaging of joint regions.
These and other objects are achieved by the methods herein disclosed for imaging joint spaces utilizing magnetic resonance imaging. Such regions of interest, or joint spaces, are typically those joint spaces found in vertebrate animals, including, but not limited to, humans, pigs, mice, horses.and the plethora of other vertebrates too numerous to list. More particularly, joint spaces are points of articulation between two or more bones, especially such a connection that allows motion. These images are generated efficiently by utilizing a magnetic resonance scanner under disclosed conditions that remove bone and marrow signals from the resultant images as well as other joint region components if so desired.
In order to acquire clinically useful images of joint spaces, the subject, a vertebrate for example, undergoing the imaging procedures disclosed herein, is injected intravenously with a blood pool contrast agent composition. The blood pool contrast agent composition is allowed to circulate throughout the subject. Subsequently, the subject, now containing the blood pool contrast agent composition, is positioned into a magnetic resonance imaging apparatus so that the area of interest, in particular a joint region, is subjected to a magnetic polarizing field. The magnetic polarizing field may be in the range of about 0.1 to 5 Tesla, preferably about 0.25 to 1.5 Tesla.
Following this step, the joint region is subjected to a variety of specific radio-frequency (rf) electromagnetic field pulse sequences that result in the selective deletion and/or imaging of particular components of the joint region in the resulting images. For example, various fluids such as synovial fluid are found in particular joints such as knee joints and inter-vertebral joints, respectively. In order to suppress these joint components"" contribution to the image, water suppression techniques can be used, as detailed below. Similarly, the methods disclosed herein may be utilized to visualize cartilage, such as hydrated cartilage as well as Lumbar disks, for example.
The previously administered blood pool contrast agent composition is used in order to suppress the contribution of bone and marrow to the image as well as blood, if so desired. The area being imaged contains the blood pool contrast agent composition which is comprised of suitably configured components that allow for the uptake of this pharmaceutical into the reticuloendothelial system. It is by the careful consideration of size and composition of these blood pool contrast agents that this composition is cleared into bone marrow as well as being able to remaining in the bloodstream. Here, by employing the magnetic resonance imaging scanner to collect fat-suppressed, T2-weighted images, in conjunction with the administration of the aforementioned blood pool contrast agent compositions, bone, marrow and fat are eliminated in the image. The image therefore displays the remaining joint space, and when combined with a series of consecutive joint space images taken under the same parameters, can be subjected to image data analysis. Such image data analysis can include various methods of computational three-dimensional projection/reconstruction, in order to provide unobstructed observation of the joint space from essentially any point of view. That is, the resultant three dimensional image may be rotated xe2x80x9cvirtuallyxe2x80x9d to provide a view of the joint space from a desired point of view, unobstructed by bone, marrow, blood vessels and fat, for example.